Many modern aircraft, as well as other vehicles and industrial processes, employ gas turbine engines for generating energy and propulsion. Such engines include a fan, compressor, combustor and turbine provided in serial fashion, forming an engine core and arranged along a central longitudinal axis. Air enters the gas turbine engine through the fan and is pressurized in the compressor. This pressurized air is mixed with fuel in the combustor. The fuel-air mixture is then ignited, generating hot combustion gases that flow downstream to the turbine. The turbine is driven by the exhaust gases and mechanically powers the compressor and fan via an internal shaft. Energy from the combustion gases not used by the turbine is discharged through an exhaust nozzle, producing thrust to power the aircraft.
Gas turbine engines contain an engine core and fan surrounded by a fan case, forming part of a nacelle. The nacelle is a housing that contains the engine. The fan is positioned forward of the engine core and within the fan case. The engine core is surrounded by an engine core cowl and the area between the nacelle and the engine core cowl is functionally defined as a fan duct. The fan duct is substantially annular in shape to accommodate the airflow from the fan and around the engine core cowl. The airflow through the fan duct, known as bypass air, travels the length of the fan duct and exits at the aft end of the fan duct at an exhaust nozzle.
In addition to thrust generated by combustion gasses, the fan of gas turbine engines also produces thrust by accelerating and discharging ambient air through the exhaust nozzle. Various parts of the gas turbine engine generate heat while operating, including the compressor, combustor, turbine, central rotating shaft and fan. To maintain proper operational temperatures, excess heat is often removed from the engine via oil coolant loops, including air/oil or fuel/oil heat exchangers, and dumped into the bypass airflow for removal from the system.
The fan includes a number of blades arranged radially from the central longitudinal axis. The blades rotate about the central longitudinal axis when in operation. Each blade has a tip located at the blade's extreme end, distal to the central longitudinal axis. As the fan rotates, each blade will pass at a distance from a point along the fan case. Certain distances, or tolerance levels, are desirable for different gas turbine engine performance characteristics. If such a tolerance level is not achieved, conditions adverse to gas turbine engine efficiency may result, including increased turbulence, internal drag or flow around the fan rather than through the fan.
Accordingly, it is important to accurately monitor the distance between the tip and the fan case to maintain tolerances. As a gas turbine engine operates, certain parts may expand due to the heat generated and absorbed. These may include fan blades, the fan case or a liner. The liner may be a part of the fan case or an independent part, and may expand along with the blades. In this manner, a desired tolerance level may be maintained. However, if a sensor for monitoring the distance between the tip and the fan case is not allowed to compensate for thermal expansion, measurement accuracy may suffer. Further, if a sensor is designed with hard leads or a prohibitive size or weight, accurate measurement may also be hindered.
Accordingly, there is a need for an improved system for monitoring tip clearance.